1. Field of the Invention
The present invention relates to a mobile communication system, a base station, a method of controlling packet transmission timing used for the mobile communication system and the base station, and a recording medium on which a program is recorded. More particularly, the present invention relates to a method of controlling timing of transmission of data by high speed downlink packet access (HSDPA) method.
2. Description of the Prior Art
In recent years, the development of multimedia-ready mobile terminals (mobile stations) such as a cellular phone has been advanced to enable handling of still images of a large data amount and short-time moving images. A need has arisen for a high-capacity high-speed data transmission method for use of such mobile devices.
As such a high-capacity high-speed data transmission method, HSDPA methods using a physical downlink shared channel (PDSCH) or a high-speed-physical downlink shared channel (HS-PDSCH) such that only the transmission speed in the downstream direction (the direction from a base station to a mobile station) is increased, and other methods have been proposed.
In a mobile communication system in which data is transmitted from a base station to a mobile station by using the above-mentioned HS-PDSCH, the quality of reception (Ec/Io (energy per chip/interfering wave power per unit frequency)) of a common pilot signal (common pilot channel (CPICH)) transmitted from each of base stations during soft handover is measured in a mobile station, and a base station controller is notified of the measurement result from the mobile station through the base station.
The base station controller identifies, on the basis of the measurement result from the mobile station, the base station with which the highest quality of reception of the common pilot signal data is achieved, and transfers to this base station data to be transmitted by HSDPA method. The base station divides the data transferred from the base station controller into blocks and transmits the divided data blocks one after another at an optimum data transmission rate according to the channel quality of the downstream line. The optimum data transmission rate is a highest possible transmission rate at which data can be transmitted while the desired error rate is ensured. Data can be transmitted at a higher rate if the channel quality is higher or the reception signal interference ratio (SIR) (signal power/(interference power+noise)) is higher in the mobile station. Since in ordinary cases the transmission power in the base station is constant for the HSDPA method, the transmission speed in the HSDPA method is inversely proportional to the interference power if noise is constant.
In the above-described conventional mobile communication system, transmission timing for transmission of data by HSDPA method is independently performed in each base station and data is transmitted from only one base station even to a mobile station located in the vicinity of a cell boundary. While data is being transmitted to this mobile station by HSDPA method, another base station having its cell boundary in the vicinity of this mobile station may transmit data to another mobile station by HSDPA method. In such a situation, interference at the mobile station in the vicinity of the cell boundary increases to reduce the reception quality. A deterioration is thereby caused in the channel quality of the mobile station in the vicinity of the cell boundary to make it difficult to perform high-speed transmission to the mobile station.
Moreover, since the mobile station notifies the base station controller of the result of measurement of the common pilot signal reception quality through the base station, and since data to be transmitted by HSDPA method is transferred from the base station controller to the base station with which the highest quality of reception of the common pilot signal is achieved, a control delay occurs before a start of data transmission by HSDPA method. Because the state of the propagation path changes even during the period of this delay, it cannot be ensured that the line for data transmission by HSDPA method provided by the base station selected by the base station controller is always highest in quality.